1. Field of the Invention
This invention relates to means for recovering hydrocarbons from a subterranean formation and more particularly to a method and means for controlling transport of fine particulate solids produced during a stimulation treatment during the subsequent production of hydrocarbons from a subterranean formation.
2. Brief Description of the Prior Art
Transport of particulate solids during the production of hydrocarbons from a subterranean formation is a continuing problem. The transported solids can erode or cause significant wear in the hydrocarbon production equipment used in the recovery process. The solids also can clog or plug the wellbore thereby limiting or completely stopping fluid production. Further, the transported particulates must be separated from the recovered hydrocarbons adding further expense to the processing.
The particulates which are available for transport may be present due to the nature of a subterranean formation and/or as a result of well stimulation treatments wherein proppant is introduced into a subterranean formation
In the treatment of subterranean formations, it is common to place particulate materials as a filter medium and/or a proppant in the near wellbore area and in fractures extending outwardly from the wellbore. In fracturing operations, proppant is carried into fractures created when hydraulic pressure is applied to these subterranean rock formations to a point where fractures are developed. Proppant suspended in a viscosified fracturing fluid is carried outwardly away from the wellbore within the fractures as they are created and extended with continued pumping. Upon release of pumping pressure, the proppant materials remain in the fractures holding the separated rock faces in an open position forming a channel for flow of formation fluids back to the wellbore.
Introduction of the proppant materials into the fracturing fluid often results in the crushing of some portion of the proppant material as it passes through the pumping and mixing equipment to enter the subterranean formation. This fine crushed material may have a particle size ranging from slightly below the size of the original proppant material to less than 600 mesh on the U.S. Sieve Series. Also, when the formation closes at the conclusion of the treatment some crushing of the proppant material may occur producing additional fines. Even when proppant crushing does not occur, the subterranean formation may itself release fines from the face of the created fractures as a result of spalling, scouring of the formation wall which causes formation particulate to be mixed with the proppant and the like. These fine formation materials also may range from formation grain size to less than 600 mesh. The fines may comprise sand, shale or hydrocarbons such as coal fines from coal degasification operations and the like. When the wellbore subsequently is produced, the fines tend to move into the proppant pack in the fracture tending to reduce the permeability of the pack. The fines also can flow with any production from the wellbore to the surface.
This undesirable result causes undue wear on production equipment and the need for separation of solids from the produced hydrocarbons. Fines flowback often may be aggravated by what is described as "aggressive" flowback of the well after a stimulation treatment. Aggressive flowback generally entails flowback of the treatment fluid at a rate of from about 0.001 to about 0.1 barrels per minute (BPM) per perforation of the treatment fluids which were introduced into the subterranean formation. Such flowback rates accelerate or force closure of the formation upon the proppant introduced into the formation. The rapid flowrate can result in large quantities of fines flowing back into the near wellbore as closure occurs causing permeability loss within the formation. The rapid flowback is highly desirable for the operator as it returns a wellbore to production of hydrocarbons significantly sooner than would result from other techniques.
Currently, the primary means for addressing the formation particulate or fines problem is to employ resin-coated proppants or resin consolidation of the proppant which is not capable of use in aggressive flowback situations. Further, the cost of resin-coated proppant is high, and is therefore used only as a tail-in in the last five to twenty five percent of the proppant placement. Resin-coated proppant is not always effective at forming a filtration bed since there is some difficulty in placing it uniformly within the fractures and, additionally, the resin coating can have a deleterious effect on fracture conductivity. Resin coated proppant also may interact chemically with common fracturing fluid crosslinking systems such as guar or hydroxypropylguar with organo-metallics or borate crosslinkers. This interaction results in altered crosslinking and/or break times for the fluids thereby affecting placement.
In unconsolidated formations, it is common to place a filtration bed of gravel in the near-wellbore area in order to present a physical barrier to the transport of unconsolidated formation fines with the production of hydrocarbons. Typically, such so-called "gravel packing operations" involve the pumping and placement of a quantity of gravel and/or sand having a mesh size between about 10 and 60 mesh on the U.S. Standard Sieve Series into the unconsolidated formation adjacent to the wellbore. Sometimes multiple particle size ranges are employed within the gravel pack. It is sometimes also desirable to bind the gravel particles together in order to form a porous matrix through which formation fluids can pass while straining out and retaining the bulk of the unconsolidated sand and/or fines transported to the near wellbore area by the formation fluids. The gravel particles may constitute a resin-coated gravel which is either pre-cured or can be cured by an overflush of a chemical binding agent once the gravel is in place. It has also been known to add various hardenable binding agents or hardenable adhesives directly to an overflush of unconsolidated gravel in order to bind the particles together. Various other techniques also are described in U.S. Pat. No. 5,492,178, the entire disclosure of which is incorporated herein by reference.
U.S. Pat. Nos. 5,330,005, 5,439,055 and 5,501,275 disclose a method for overcoming the difficulties of resin coating proppants or gravel packs by the incorporation of a fibrous material in the fluid with which the particulates are introduced into the subterranean formation. The fibers generally have a length ranging upwardly from about 2 millimeters and a diameter of from about 6 to about 200 microns. Fibrillated fibers of smaller diameter also may be used. The fibers are believed to act to bridge across constrictions and orifices in the proppant pack and form a mat or framework which holds the particulates in place thereby limiting particulate flowback. The fibers typically result in a 25 percent or greater loss in permeability of the proppant pack that is created in comparison to a pack without the fibers.
While this technique may function to limit some flowback, it fails to secure the particulates to one another in the manner achieved by use of resin coated particulates.
U.S. Pat. No. 5,551,514 discloses a method for sand control that combines resin consolidation and placement of a fibrous material in intimate mixture with the particulates to enhance production without a gravel pack screen.
It would be desirable to provide a method which will bind greater numbers of fines particles to the proppant material in such a manner that it further assists in preventing movement or flowback of particulates from a wellbore or formation without significantly reducing the permeability of the particulate pack during aggressive flowback of treatment fluids.